1. Field of the Invention
The present invention relates to an optical apparatus for focusing a light beam for use in an optical disk apparatus and, particularly, in a magnetooptical disk apparatus.
2. Description of the Prior Art
In an optical disk apparatus or a magnetooptical disk apparatus, information is stored in an optical memory or information memory medium, such as an optical disk or a magnetooptical disk and is read out thereof to reproduce the same by using an optical pick-up means.
A conventional optical apparatus for focusing a light beam, for use in the optical or magnetooptical disk apparatus, includes a U-shaped base plate and a holding or movable block, movably mounted thereto, for retaining an object lens, as disclosed in Japanese Utility Model Laid-Open Specification No. 59-221840. A focusing coil for driving the movable block in the focusing direction is wound around the side surfaces of the movable block, and two pairs of D-shaped tracking coils for driving the movable block in the tracking direction are attached to the two opposite side surfaces of the movable block. A pair of yokes are provided on the base plate.
A printed circuit board is attached to one side surface of the movable block, and the electric wires of the focusing and tracking coils are connected to the printed circuit board. Another printed circuit board is mounted to one side of the U-shaped base plate, and four electric wire-covered rubber tubes extend between the two printed circuit boards. The U-shaped base plate is made of a magnetic material, and a pair of magnets are mounted to the opposite sides thereof to form a magnetic field, in which the focusing and tracking coils are positioned.
In this embodiment, when electric current is supplied to the focusing coil or the tracking coils through the electric wires, the movable block and the object lens mounted thereto are driven in the focusing or tracking direction by the interaction between the focusing coil or tracking coil and the magnets. As a result of the energization, the wires are flexed and the object lens is displaced in the focosing and tracking directions. Thereby the object lens is held at the track position where the recording or reproducing of information is carried out, even when the information memory medium is distorted or off center.
Since the object lens is displaceably supported by the four wires either in the focusing direction or in the tracking direction in the conventional optical apparatus, the optical apparatus can not stand still quickly due to the inertial force even when the optical apparatus is moved by a transport mechanism in the tracking direction and it is stopped.
As a result, when the optical apparatus is moved by the transport mechanism in the tracking direction, the supporting unit, which is supported by the wires, swings in the tracking direction, thereby resulting in a problem that information can not be recorded or can not be reproduced until the swinging damps out.
Moreover, in the conventional optical apparatus, when the object lens is being displaced in the tracking direction by energizing the tracking coil, part of one side of the focusing coil does not often exist between the magnet and the yoke.
As a result, when the focusing coil is energized in a condition that the object lens is being displaced in the tracking direction, the center of the driving force which is generated by the focusing coil fails to coincide with the center of gravity position of the object lens. Consequently, a moment is generated by impartation of an unequal energizing force to the object lens holding block due to the off-center effect; and the object lens is rotated by the moment, thereby resulting in a coma (aberration) at the focal point of the object lens whereby satisfactory data can not be obtained.